The present invention relates to an optical disc discriminating system, in particular to a system for discriminating a read-only disc such as a DVD-ROM (Digital Video Disc-Read Only Memory) and a writable disc such as a DVD-R (Digital Video Disc-Recordable).
FIG. 7 indicates a conventional system for discriminating the types of different optical discs to determine if it is a read-only disc such as a CD-ROM (Compact Disc-Read Only Memory) or a writable disc such as a CD-R (Compact Disc-Recordable).
In FIG. 7, a reference character D is an optical disc to be discriminated, 101 is an optical pickup, 102 is a spindle motor for rotating the optical disc D, 103 is a motor control circuit for controlling the rotation of the spindle motor 102, 104 is a head amplifier, 105 is a servo circuit, 106 is an information pit detector, 107 is a CPU (Central Processing Unit).
The optical disc D to be discriminated is mounted on a turntable (not shown) of the spindle motor 102 which rotates in accordance with a control signal from the motor control circuit 103. The motor control circuit 103, in accordance with a command from the CPU 107, compares a pulse signal (FG) having a frequency proportional to the motor rotation speed, with another pulse signal (FG) indicating a rotation speed previously set by a setting section 171 of the CPU 107, so as to produce a frequency difference between these two pulse signals. Then, a control signal for rendering the frequency difference to become zero is fed to the spindle motor 102. In this manner, a servo loop is formed for controlling the rotation of the optical disc D. Therefore, the optical disc D which is to be discriminated later, is controllably rotated at a speed previously set by the CPU 107.
On the other hand, a laser beam irradiating from the optical pickup 101 is reflected on the recording surface of the optical disc D (to be discriminated), a reflection diffraction light bearing the information from the optical disc D is received by a photodetector (not shown) of the optical pickup 101. Then, the reflection diffraction light is converted into an electric signal to be fed to the head amplifier 104. The head amplifier 104 processes the above electric signal so as to first produce and then amplify a RF signal and an error signal to be used in focusing control/tracking control. The amplified signals are fed to the servo circuit 105 and the information pit detector 106. The servo circuit 105, in view of an error signal applied hereto and in accordance with an command from the CPU 107, produces a servo control signal for controlling a slid motor (not shown) which may effect a radial movement of the optical pickup 101, a focus actuator (not shown) and a tracking actuator (not shown) both of which are contained in the optical pickup 101. Further, the servo circuit 105 produces a tracking lock signal (indicating a condition determined by a tracking control operation after the tracking servo loop is closed) to the information pit detector 106. The information pit detector 106 extracts an envelope of the RF signal, and determines whether the envelope has a higher level than a predetermined level. If the envelope has a higher level than a predetermined level, the pit detector 106 produces a signal (indicating that the optical disc D has formed thereon information pits) to the CPU 107.
The types of different optical discs may be discriminated with the use of the system of FIG. 7 in a manner which will be described in detail below.
At first, the CPU 107 gives a command to the servo circuit 105 to drive a slide motor (not shown) so as to move the optical pickup 101 in a radial direction to a position corresponding to a TOC (Table of Contents) area on the optical disc D. Then, the CPU 107 produces a command signal to the motor control circuit 103 to cause the optical disc D to rotate in a predetermined speed. Subsequently, a laser beam is emitted from the optical pickup 101, a focus control and a tracking control are effected by means of the servo circuit 105 such that the laser beam is converged on the information recording surface of the optical disc D and trace along information tracks thereon. After that, if the pit detector 106 has produced a signal indicating that the optical disc D has information pits formed thereon, it is determined that the optical disc D is a read-only CD or a finalized CD-R which does not contain any unrecorded portions. On the other hand, if the pit detector 106 does not produce such a signal, it. is determined that the optical disc D is a CD-R which contains un-recorded portions.
A principle for discriminating the types of different optical discs in the above system of FIG. 7 may be explained as follows.
Namely, a CD-ROM or a finalized CD-R, which is a read-only optical disc, has formed in its TOC area a lot of pits representative of TOC information. Thus, when tracks are traced a RF signal will be read out which has an envelope whose level will be higher than a predetermined level, thereby producing a signal indicating that pits have been detected. On the other hand, a CD-R which contains unrecorded portions has not formed in its TOC area any pits representative of TOC information (because contents to be recorded have not yet been finally decided and thus the disc itself has not been finalized), although when tracks are traced a RF signal will be read out, an envelope of the RF signal will have a level which equals to zero, hence failing to produce any signal indicating that pits have been detected.
Recently, there has been developed a further advanced optical disc such as a read-only DVD-ROM (Digital Video Disc-Read Only Memory) which has a much larger information recording capacity and a higher recording density than a read-only CD. Also, there have been developed further advanced writable optical discs including a DVD-R (Digital Video Disc-Recordable), DVD-RAM (Digital Video Disc-Random Access Memory), which have an identical signal format as the read-only DVD-ROM but can optionally record movies or music a user has selected. Further, it has been suggested that a general system be designed to enable the DVD, DVD-R and DVD-RAM to be used so as to perform recording or reproducing on only one recording/reproducing device. With such a general system, it is necessary to discriminate the types of different optical discs so as to perform a desired changeover into an equivalent circuit having a characteristic corresponding to an optical disc to be used for recording or reproducing.
However, as shown in FIG. 8, when pit height or groove depth of an optical disc is .lambda./4n (.lambda. is a wave length of a laser beam, n is an index of refraction of a substrate of an optical disc), an RF signal from the optical disc will reach a maximum level, thus a C/N (Carrier to Noise) of the RF signal is in its optimum condition. On the other hand, a push-pull signal (tracking error signal) indicating a difference between two outputs from a two-section (equally divided) photodetector will become maximum when pit height or groove depth is .lambda./8n, but will become minimum when the height or the depth is .lambda./4n.
Where an optical disc is a CD-ROM, a pit height or a groove depth is designed to be .lambda./6n, so as to obtain a RF signal having a level equal to or higher than a predetermined value, and at the same time to surely perform a tracking control using a push-pull signal.
Where an optical disc is a DVD-ROM, since its recording density is seven times as high as a CD-ROM, an optical pickup, which is to be used in the above-described general system, has to be designed with an extremely high precision so as to meet the requirements prescribed in MTF (Modulation Transfer Function). In order to reduce a burden on the designing of such an optical pickup which is for use in the general system, the height of the pits formed on the DVD-ROM is designed to be .lambda./4n so as to obtain a RF signal having an optimum signal quality. Consequently, as a tracking control on the DVD in a comprehensive general system, it is impossible to employ a Push-pull Method, rather it is generally possible to employ a Phase Difference Method.
On the other hand, where an optical disc is a writable disc such as a DVD-R or a DVD-RAM, the recording of information on such an optical disc usually requires that a laser beam should have a high output power. Thus, it is preferable that a laser beam being emitted from a light source be used as efficiently as possible. Accordingly, as a tracking control on the DVD-R, it is preferable to employ a Push-pull Method which does not need to divide a laser beam, or allows a divided beam (for use in tracking control) to be much smaller than a beam for use in recording information. In contrast, the Phase Difference Method is not suitable for a DVD-R or a DVD-RAM, although the method allows a divided beam (for use in tracking control) to be smaller than a beam for recording information, because there are no pits formed on an unrecorded portions of a writable disc, making it impossible to obtain error signal. Further, in order to make a good trade-off (a balance) between the push-pull signal and the RF signal, a pit height or a groove depth is required to be approximately .lambda./5n so as to ensure that both the push-pull signal and RF signal reach or exceed their predetermined levels.
As is understood from the above description, a tracking control using Push-pull Method can not be used for a read-only optical disc such as a DVD-ROM, on the other hand, a tracking control using Phase Difference Method can not be used for a writable optical disc such as a DVD-R or a DVD-RAM.
Since there is no common tracking control which is useful for both a DVD-ROM and a DVD-R (or DVD-RAM), a conventional discriminating method can not be used for discriminating a DVD-ROM from a DVD-R (or DVD-RAM), because a conventional method requires a prerequisite that a tracking control be performed to render a laser beam to correctly trace tracks on an optical disc.
Moreover, in discriminating a DVD-R or a DVD-RAM having unrecorded portions, a conventional method (which discriminates a DVD-R or a DVD-RAM by confirming if there are information pits formed on the TOC area of an optical disc) fails to be used therefor because there are not information pits formed on the TOC area of a DVD-R or a DVD-RAM